A known image forming apparatus includes an image forming unit configured to form an image on a recording medium, e.g., a sheet of paper. In the image forming apparatus, an image is formed on the sheet, and then thermally fixed onto the sheet. During image fixing, the sheet may crease. To avoid creasing on the sheet, it has been proposed to provide an area where no image is formed (hereinafter referred to as a non-image formation area) in each of a leading end and a trailing end of a recording medium when duplex printing is performed. Duplex printing includes an image forming unit forming an image on a first side of the recording medium, and forming an image on a second side of the recording medium opposite to the first side upon the recording medium being fed again to the image forming unit.
Moreover, recording media are more likely to crease with higher humidity. After the image forming unit forms an image on a first side of a recording medium, a crease may form on the recording medium during further image fixing due to potentially higher humidity. When the image forming unit forms an image on a second side of the recording medium having such a crease, the image may not be formed well in a portion around the crease. Thus, during image formation on the second side of the recording medium having higher humidity than that on the first side, a crease may form on the recording medium, especially near a trailing end of the recording medium in a feed direction, where it is more likely that a crease will form, and thus, there is a possibility that the image might not be formed desirably in the trailing end.